Apparatus and method for reception and transmission of control channels

ABSTRACT

A base station can operate a first serving cell on a licensed carrier frequency and a second serving cell on an unlicensed carrier frequency. The base station can perform a listen before talk (LBT) on the unlicensed carrier frequency for a first time duration. The base station can determine that the unlicensed carrier frequency is free. The base station can transmit an activation command on the first serving cell for receipt by a UE, the activation command being configured to activate the UE to receive transmissions from the base station on the second serving cell. The base station can transmit after determining that the unlicensed carrier frequency is free, a reserve transmission on the unlicensed carrier frequency at least until the base station begins transmission of the activation command on the first serving cell.

TECHNICAL FIELD

The following disclosure relates to wireless telecommunicationtransmissions of Long Term Evolution (LTE) type communication signals.In particular, the following disclosure relates to approaches fortruncating transmissions in the LTE subframe signal and signalingoptions for the transmission of control channels and reference signals.

BACKGROUND

LTE Advanced is being considered for deployment in the unlicensedspectrum in the 5 GHz band. To do so, licensed spectrum in the range 400MHz to 3.8 GHz and spectrum in the unlicensed spectrum bands would beaggregated together similar to the way that the licensed spectrum bandsare treated in standard LTE Advanced carrier aggregation (CA).Regulatory authorities around the world have defined or are defining awide variety of requirements so that various systems can inter-operatewith incumbent users in the unlicensed bands, notably including WiFideployments.

For LTE Advanced (LTE Rel10-12) carrier aggregation (CA) or DualConnectivity, an eNB (E-Utran Node B) may be used. An eNB is connectedto a mobile phone network that communicates with mobile handsets (UEs)and that configures a secondary serving cell (Scell) to the UE toprovide additional frequency resources (i.e., a secondary carrier or asecondary component carrier (CC)) for communication in addition to theprimary serving cell (Pcell). The Scell is typically activated for a UEat the onset of a data burst transmission for the UE and deactivatedafter the transmission is complete. Activation is done by using anactivation command MAC layer control element (MAC CE). Deactivation isdone either upon expiry of a deactivation timer or by using an explicitdeactivation command MAC CE. Additionally, a UE operating in LTEAdvanced (LTE Rel12) network can expect at least discovery signals fromthe eNB on the Scell carrier while the carrier is deactivated. Afterreceiving an activation command for the Scell, the UE expects cellspecific reference signal (CRS) transmission from the eNB on the Scellcarrier from the subframe where activation command is received till thesubframe where the Scell is deactivated.

For further Advanced LTE (LTE Rel13) Scell, operation on unlicensedcarriers continues to be studied. The initial focus of the studiesappears to be Scell operation via a CA mechanism. However, it has beenperceived that some of the procedures identified for CA can also bereused for dual connectivity (i.e., when the Scell and Pcell belong todifferent cell groups).

Due to regulatory requirements, and due the need to for Advanced LTE toco-exist with other wireless systems (e.g., Wi-Fi), LTE devices (i.e.,UEs and eNBs) need to take the following issues into account whileoperating on an unlicensed carrier frequency.

First, before transmitting on an unlicensed carrier, the LTE devices(e.g., eNB) typically have to check whether the carrier (i.e., thecarrier frequency) is busy using some form of ‘listen before talk’ (LBT)mechanism, then an LTE device can begin transmissions only if thecarrier is free. LBT typically comprises measuring the energy on thecarrier (sometimes referred to as sensing) for a short duration (e.g., 9us or 20 us) and determining whether the measured energy is less than athreshold (e.g., −82 dBm or −62 dBm). If the energy is less than thethreshold, the carrier is determined to be free. Some examples of LBTinclude the CCA-ED (clear channel assessment-energy detect) and CCA-CS(clear channel assessment-carrier sense) mechanisms defined in IEEE802.11 specifications, CCA mechanisms specified in ETSI EN 301 893specification etc.

And, second, transmissions on the carrier typically also have to followdiscontinuous transmission requirements (DCT requirements), i.e., theLTE device can continuously transmit for X ms (e.g., X=4 ms as per someregulations, up to 13 ms for some other regulations), after which it hasto cease transmission for some duration (sometimes referred as idleperiod), perform LBT, and reinitiate transmission only if LBT issuccessful. The device may perform LBT towards the end of the idleperiod.

As such, what is needed is a modification to certain transmissions ofLTE signals and on various channels that enable efficient wirelessnetwork operation in both the licensed and unlicensed carrier spectrums.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding, reference is now made to thefollowing description taken in conjunction with the accompanyingDrawings in which:

FIG. 1 illustrates a communications system in accordance with variousinvention embodiments;

FIG. 2 illustrates a possible configuration of a computing system to actas a base station;

FIG. 3 is a block diagram of an embodiment of a telecommunicationapparatus or electronic device that operates as a User Equipment (UE);

FIG. 4A illustrates an example of a subframe structure having a subframewith a truncated portion used for communication between a base stationand a UE;

FIG. 4B illustrates and example of the (n) subframe of FIG. 4A showingS0 through s13 OFDM symbols during the subframe time duration;

FIG. 5 illustrates another subframe structure having a truncated portionduring a terminal portion of one subframe and during a first slotportion of the following subframe;

FIG. 6 illustrates another subframe structure having a truncated portionduring a terminal portion of one subframe and during a portion of afirst slot of the following subframe; and

FIG. 7 illustrates and example of Scell operation on an unlicensedcarrier frequency.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with referenceto the accompanying drawings, in which various embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey pertinent aspects of the invention to those skilled in the art.

As will be explained and discussed in more detail below, variousembodiments provide enhancements or modifications to the creation andreception of LTE signals and channels in order to enable operation inboth licensed and unlicensed frequency spectrums.

In one embodiment a method is provided for a UE to receive a subframestructure as a communication from a base station, such as an eNB. Themethod comprises the UE receiving a subframe in the subframe structure.The received subframe comprises a first time slot comprising a firstplurality of OFDM symbols and a second time slot comprising a secondplurality of OFDM symbols, wherein the first time slot and the secondtime slot are non-overlapping time slots. The UE monitors for downlinkcontrol information in a first set of control channel elements (CCEs)with the first n2 (1≤n2≤3) OFDM symbols of the first time slot. The UEalso monitors for downlink control information in a second set ofcontrol channel elements with the first n2 (1≤n2≤3) OFDM symbols of thesecond time slot. The monitoring for downlink control information may bedone in parallel (i.e., at the same time) or in series with themonitoring for downlink control information. The UE then determinesresulting downlink control information from the monitoring of the firstand second sets of CCEs.

In some embodiments the first plurality of OFDM symbols consists ofseven OFDM symbols and the second plurality of OFDM symbols alsoconsists of seven ODFM symbols. In some embodiments, the downlinkcontrol information in the second set of CCEs is determined to be theresulting downlink control information when the subframe is a subframewith truncation, which includes at least one of the first plurality ofOFDM symbols in the first time slot.

Additionally, in various embodiments, receiving of the subframestructure further comprises receiving the subframe from a serving cellin an operating channel of an unlicensed frequency band. Variousembodiments may operate such that the resulting downlink controlinformation from either the first or the second set of CCEs compriseresource allocation information prescribing an allocation of resourcesfor Physical Downlink Shared Channel (PDSCH) reception.

In embodiments wherein the resulting downlink control information isdetermined to be from the monitoring for downlink control information inthe second set of CCEs, then the second set of CCEs comprises resourceallocation information allocating resources in the second plurality ofOFDM symbols in the second time slot. In some embodiments the monitoringfor downlink control information in the second set of CCEs is only donewhen the monitoring for control information in the first set of CCEs inthe first slot fails to find any downlink control information therein.Alternatively, in some embodiments wherein the determined resultingdownlink control information is from the monitoring for controlinformation in the first set of CCEs within the OFDM symbols in thefirst time slot, the resource allocation information can compriseresource allocation information prescribing an allocation of resourcesin another plurality of OFDM symbols that are in a second subframe thatoccurred immediately prior to the subframe from which the resourceallocation information was in.

In various embodiments the determined resulting downlink controlinformation is from monitoring downlink control information in the firstset of the CCEs within the first OFDM symbols of the first time slot andwherein the resulting downlink control indication comprises anindication value that indicates that the subframe is a subframe controlindication with truncation and indicates a number of symbols in thetruncated portion of the subframe.

Another embodiment provides a method for a base station to create asubframe structure used in communication between the base station and aUE wherein the subframe structure comprises a plurality subframes thateach have a subframe duration. The method for the base station to createthe subframe structure comprises the base station operating a servingcell on a carrier frequency. The base station performs a listen beforetalk (LBT) on the carrier frequency for a LBT time duration. The LBTtime duration at least partially overlaps a terminal portion of asubframe duration of first subframe within the plurality of subframes inthe subframe structure. The base station then determines where aposition of a starting OFDM symbol for transmitting a control channel ina second subframe of the plurality of subframes should be placed basedon the LBT time duration of the LBT and wherein the subframe duration ofthe second subframe occurs immediately after the subframe duration ofthe first subframe.

In various embodiments, the base station transmits the control channelstarting from a determined position of the starting OFDM symbol in thesecond subframe when a result of the LBT time duration indicates thatthe carrier frequency is unoccupied during the terminal part of theterminal portion of the first subframe duration. Alternatively, the basestation may transmit the control channel starting at the eighth OFDMsymbol (or at the beginning of a second slot) of the second subframewhen the result of the LBT time duration indicates that the carrierfrequency is occupied during the terminal portion of the first subframeduration.

In yet another embodiment, a method for base station is provided whereinthe base station creates a subframe structure used in communicationbetween the base station and a user equipment (UE). The subframestructure comprises a plurality of subframes that each have a subframeduration. This embodiment, the base station operator serving cell on acarrier frequency. The base station performs a listen before talk (LBT)on the carrier frequency for a time duration during at least a firstportion of a first subframe wherein the first portion of the firstsubframe occurs during a terminal portion of the first subframe andpossibly during at least a second portion of a second subframe, whereinthe second portion of the second sub frame occurs at the beginning ofthe second subframe and wherein the second subframe occurs immediatelyafter the first subframe. The base station then determines a position ofa starting OFDM symbol for transmitting a control channel in the secondsubframe of the plurality of subframes based on the time duration of theLBT. Also, this embodiment the second subframe has a second subframeduration that occurs immediately after the subframe duration of thefirst subframe.

In various variations of this embodiment the position of the startingOFDM symbol may be determined by determining that the position of thestarting OFDM symbol is at an initial OFDM symbol position in the secondsubframe when the result of the LBT indicates that the carrier frequencybecame unoccupied during a terminal part of the first portion of thefirst subframe. Determining the position of the starting OFDM symbol mayalso include determining that the position of the starting OFDM symbolis that and eighth OFDM symbol position in the second subframe when theresult of the LBT that the carrier frequency became unoccupied duringthe second portion of the second subframe. Here the second portion ofthe second subframe may comprise the initial through the seventh OFDMsymbol positions of the second subframe and wherein the second subframecomprises fourteen OFDM symbol positions. In some embodiments the basestation also transmits the control channel starting from the determinedposition of a starting OFDM symbol.

Additionally, in various embodiments the base station performs physicaldownlink shared channel (PDSCH) transmissions to the UE in the firstsubframe using the first UE-specific reference signal pattern when theduration of the first portion is not greater than the duration of thelast three OFDM symbols of the first subframe. The base station may alsoperform PDSCH transmission to the UE in the first subframe using asecond UE-specific reference signal pattern when the duration of thefirst portion is greater than the duration of the last three OFDMsymbols of the first subframe.

In some embodiments the first UE-specific reference signal pattern is aUE-specific reference signal pattern used for the LTE frame structuretype 2 (TDD) special subframe configurations 3, 4, 8, or 9; And whereinthe second UE-specific reference signal pattern is a UE-specificreference signal pattern used for LTE frame structure type 2 (TDD)special subframe configurations 1, 2, 6 or 7.

Yet another embodiment of the invention provides a method for basestation to make a transmission between the base station and a UE on anunlicensed carrier frequency. Here an exemplary method comprises thebase station operating a first serving cell on a licensed carrierfrequency and a second serving cell on an unlicensed carrier frequency.The base station performs a listen before talk (LBT) on the unlicensedcarrier frequency for a first time duration. The base station thendetermines whether the unlicensed carrier frequency is free based on theLBT results. The base station may then transmit an activation command onthe first serving cell for receipt by the UE. The activation command isconfigured to activate the UE so that it receives transmissions from thebase station on the second serving cell. The base station may alsotransmit, after determining that the unlicensed carrier frequency isfree, a reserve transmission on the unlicensed carrier frequency atleast until the base station begins transmission of the activationcommand on the first serving cell.

In some embodiments the base station prepares the activation commandwhile transmitting the reserve transmission on the unlicensed carrierfrequency. Additionally, in some embodiments, the base station alsoschedules data to be transmitted to the UE on the unlicensed carrierfrequency for determined number of subframes. The base station willdiscontinue transmission of scheduled data to the UE after determinednumber of subframes and then perform another LBT on the unlicensedcarrier frequency for a second time duration.

In yet another embodiment, a UE is configured to receive a subframestructure that is used for communication from a base station to the UE.In this embodiment the UE comprises a transceiver that's configured toreceive a subframe in a subframe structure, where in the subframestructure comprises a first time slot comprising a first plurality oforthogonal frequency division multiplexed (OFDM) symbols and a secondtime slot comprising a second plurality of OFDM symbols, wherein thefirst time slot and the second time slot are non-overlapping. The UEfurther includes a processor that is configured to monitor for downlinkcontrol information in a first set of control channel elements (CCEs)within the first (m1) (1≤m1≤3) OFDM symbols of the first time slot,where (m1) is an integer. The processor is further configured to monitorfor downlink control information in a second set of control channelelements (CCEs) within the first (m2) (1≤m2≤3) OFDM symbols in thesecond time slot, where (m2) is an integer. Additionally the processoris also configured to determined downlink control information from oneof the first or second sets of CCEs.

FIG. 1 illustrates a communication system 100 including a network 102, abase station 104, such as an eNB, and a user equipment (UE) 106. Thebase station may also be referred to as a base unit, an access point(AP), access terminal (AT), Node-B (NB), enhanced Node-B (eNB), relaynode, home eNB, pico eNB, femto eNB or by other present or futureterminology used in the art for a base station derivation. Variouscommunication devices may exchange data or information through thenetwork 102. The network 102 may be an evolved universal terrestrialradio access (E-UTRA) or other type of telecommunication network. Anetwork entity, such as the base station 104, may assign a UE identifier(UEID) to the UE 106 when the UE 106 first joins the network 102. Forone embodiment, the base station 104 may be a distributed set of serversin the network 102. The UE 106 may be one of several types of handheldor mobile communication devices, such as, a mobile phone, a laptop, or apersonal digital assistant (PDA). In one embodiment, the UE 106 may be awireless local area network capable device, a wireless wide area networkcapable device, or any other wireless device.

FIG. 2 illustrates a possible configuration of a computing system to actas the base station 104. The base station 104 may include aprocessor/controller 210, a memory 220, a database interface 230, atransceiver 240, input/output (I/O) device interface 250, and a networkinterface 260, connected through bus 270. The base station 104 mayimplement any operating system, such as Microsoft Windows®, UNIX, orLINUX, for example. Client and server software may be written in anyprogramming language, such as C, C++, Java or Visual Basic, for example.The server software may run on an application framework, such as, forexample, a Java® server or .NET® framework.

The controller/processor 210 may be any programmable processor. Variousembodiments of the disclosure may also be implemented or partiallyimplemented on a general-purpose or a special purpose computer, aprogrammed microprocessor or microcontroller, peripheral integratedcircuit elements, an application-specific integrated circuit or otherintegrated circuits, hardware/electronic logic circuits, such as adiscrete element circuit, a programmable logic device, such as aprogrammable logic array, field programmable gate-array, or the like. Ingeneral, any device or devices capable of implementing the decisionsupport method as described herein may be used to implement the decisionsupport system functions of this disclosure.

The memory 220 may include volatile and nonvolatile data storage,including one or more electrical, magnetic or optical memories such as arandom access memory (RAM), cache, hard drive, or other memory device.The memory may have a cache to speed access to specific data. The memory220 may also be connected to a compact disc-read only memory (CD-ROM),digital video disc-read only memory (DVD-ROM), DVD read write input,tape drive, solid state drive or other removable memory device thatallows media content to be directly uploaded into the system. Data maybe stored in the memory 220 or in a separate database (not specificallyshown). The database interface 230 may be used by thecontroller/processor 210 to access the database. The database maycontain any formatting data to connect the UE 106 to the network 102.The transceiver 240 may create a data connection with the UE 106. Thetransceiver 240 may configure a Physical Downlink Control Channel(PDCCH) and a Physical Uplink Control Channel (PUCCH) between the basestation 104 and the UE 106.

The I/O device interface 250 may be connected to one or more inputdevices that may include a keyboard, mouse, pen-operated touch screen ormonitor, voice-recognition device, or any other device that acceptsinput. The I/O device interface 250 may also be connected to one or moreoutput devices, such as a monitor, printer, disk drive, speakers, or anyother device provided to output data. The I/O device interface 250 mayreceive a data task or connection criteria from a network administrator.

The network connection interface 260 may be connected to a communicationdevice, modem, network interface card, a transceiver, or any otherdevice capable of transmitting and receiving signals from the network106. The network connection interface 260 may be used to connect aclient device to a network. The network connection interface 260 may beused to connect the teleconference device to the network connecting theuser to other users in the teleconference. The components of the basestation 104 may be connected via an electrical bus 270, for example, orlinked wirelessly.

Client software and databases may be accessed by thecontroller/processor 210 from memory 220, and may include, for example,database applications, word processing applications, as well ascomponents that embody the decision support functionality of the presentdisclosure. The base station 104 may implement any operating system,such as Microsoft Windows®, LINUX, or UNIX, for example. Client andserver software may be written in any programming language, such as C,C++, Java or Visual Basic, for example. Although not required, thedisclosure is described, at least in part, in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by the electronic device, such as a general purpose computer.Generally, program modules include routine programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Moreover, those skilled in theart will appreciate that other embodiments of the disclosure may bepracticed in network computing environments with many types of computersystem configurations, including personal computers, hand-held devices,multi-processor systems, microprocessor-based or programmable consumerelectronics, network PCs, minicomputers, mainframe computers, and thelike.

FIG. 3 illustrates in a block diagram of one embodiment of atelecommunication apparatus or electronic device to act as the UE 106.The UE 106 may be capable of accessing the information or data stored inthe network 102. For some embodiments of the disclosure, the UE 106 mayalso support one or more applications for performing variouscommunications with the network 102. The UE 106 may be a handhelddevice, such as, a mobile phone, a laptop, a personal digital assistant(PDA) smart phone or other multi-function communication device. For someembodiments, the UE 106 may be WiFi capable device, which may be used toaccess the network 102 for data or by voice using VOIP and WiFi carrierfrequency spectrums.

The UE 106 may include a transceiver 302, which is capable of sendingand receiving data over the network 102. The UE 106 may include aprocessor 304 that executes stored programs or applications that may,among other things, monitor, control and interact with the othercomponents of the UE 106. The UE 106 may also include a volatile memory306 and a non-volatile memory 308 which are used by the processor 304.The UE 106 may include a user interface 310 that may comprise user inputelements such as a keypad, display, touch screen, and the like. The userinterface 310 may also include user output elements that may comprise adisplay screen and/or vibration and illumination indicators. The UE 106may also include an audio interface 312 that may comprise elements suchas a microphone, earphone, and speaker. The UE 106 also may include acomponent interface 314 to which additional elements may be attached,for example, a universal serial bus (USB) interface. The UE 106 mayinclude a power supply 316. In addition, the UE may be incorporated as aperipheral or integral part of a larger system such as a transportationvehicle, building, entertainment center, kiosk, or gaming device to namea few.

In Long Term Evolution (LTE) communication systems, physical layersignals and channels (e.g., a control channel like a physical downlinkcontrol channel (PDCCH), enhanced physical downlink channel (EPDCCH); adata channel like a physical downlink shared channel (PDSCH); referenceand synchronization signals like a primary synchronization signal (PSS),secondary synchronization signal (SSS), cell-specific reference signal(CRS), channel state information reference signal (CSI-RS), anddiscovery signals are transmitted by a base station 104 using orthogonalfrequency-division multiplexed (OFDM) symbols. For normal cyclic prefix(CP) operation, the OFDM symbols are of −71 us in duration. Seven OFDMsymbols may comprise a 0.5 ms slot and two slots may comprise a 1 ms LTEsubframe. Therefore, an example of an LTE subframe comprises 14 OFDMsymbols.

In order to enable efficient secondary serving cell (Scell) operation onan unlicensed carrier frequency, it is beneficial for the base stationor eNB 104 to not use some of the symbols or portions of the symbols in(i.e., to truncate some of the symbols or portions of the symbols) thetransmission of physical layer signals or channels in some of thesubframes such that they comprise less than 14 symbols, but stillutilize the truncated portion of the subframe (i.e., the remainingportion of the subframe where the symbols are not being used) to performa listen before talk (LBT) function or as an idle period.

For example, and referring to FIG. 4A, a subframe structure 400 used forcommunication between an eNB 104 and a UE 106 is shown. In this example,each subframe 402 comprises 14 symbol positions (s1-s14) shown in FIG.4B. In some embodiments, after every continuous transmission of 2subframes (e.g., subframes (n) and (n+1)), the eNB can truncate thetransmission of the 3^(rd) subframe (n+2) 406 to be 11 symbols long anduse the remaining truncated portion 404 (i.e., 3 truncated symbols) toperform LBT. Alternately in other embodiments, after every continuoustransmission of, for example, 5 subframes, the eNB can truncate thetransmissions of, for example, the 6^(th) subframe to 9 symbols and usethe remaining 5 symbols to perform LBT. Alternatively, in otherembodiments, after every continuous transmission of, for example, 11continuous subframes, the eNB can truncate the transmission of the12^(th) subframe to be, for example, 5 symbols long and use theremaining truncated portion of 9 symbols to perform LBT. These exampletransmission approaches may be consistent with requirements for loadbased equipment transmission in ETSI EN 301 893 specification.

In various embodiments, the eNB 104 may use some or the entire truncatedportion 404 of the subframe with truncation 406, for example (n+2), toperform LBT. The eNB 104 may also use some part of the truncated portion404 of the subframe with truncation 406 as an idle period (e.g., theduration of the idle period may be similar to the DIFS period describedin IEEE 802.11 specifications). After the eNB 104 performs LBT anddetermines that the carrier is free, the eNB 104 may also use some partof the truncated portion 404 of the subframe with truncation 406 totransmit a ‘carrier holding signal’ or ‘carrier busy signal’ until thenext LTE pilot/data/control transmission can begin to ensure that otherdevices sensing the carrier detect that the carrier is occupied by theeNB and defer their transmissions (not specifically shown). The ‘carrierholding signal’ may be transmitted with the same power level or PSDlevel as the subsequent LTE pilot/data/control transmission. The‘carrier holding signal’ may be a narrowband signal or a wideband signaland may/may not be centered within the operating channel on theunlicensed carrier frequency.

From a UE 106 perspective, it is beneficial for the UE 106 to be able todetermine or to be provided information defining which of the receivedsubframes in the subframe structure 400 are subframes with truncatedtransmissions 406 and the number (m) of OFDM symbols truncated in thetruncated portion 404 of the subframes with truncation 406. If thisinformation is not made available to the UE, the UE may attempt toreceive the signals/channels transmitted by the eNB in a manner thatassumes that the subframes therein do not have any truncation, whichwill lead to estimation/decoding errors that will cause retransmissionsand inefficient carrier usage.

Subframes with truncation 406 are typically done only after continuoustransmission of a certain number of subframes 402. The number ofsubframes continuously transmitted prior to a subframe with truncation406 depends on the transmission activity of the eNB 104, which in turndepends on the data arrival patterns for various UEs served by the eNB.Given this, the subframes with truncation 406 may not follow a periodicpattern in some embodiments. Further, the subframes that are subframeswith truncation may also be impacted by the variability of the durationof the idle period or LBT observation period due to, e.g., randombackoff when the carrier is detected as being occupied. Also, the numberof symbols in the truncated portion 404 in a given subframe withtruncation 406 can vary depending on operating parameters chosen by theeNB 104. After considering these aspects, the following approaches arepossible techniques or methods for providing signaling to a UE 106 toindicate which subframe(s) is a subframe with truncation and whatportion of the subframe(s) with truncation is the truncated portion.

Still referring to FIG. 4A, a first example of an approach for signalinga UE 106 to indicate that there are subframes with truncation is for theUE to be configured by higher layers (e.g., a radio research control(RRC) message) with a set of values wherein each value corresponds to anumber of truncated symbols at the end of a subframe with truncation.When the UE 106 is assigned a PDSCH in a subframe and when that subframeis a subframe with truncation 406, the number of symbols (m) in thetruncated portion 404 of that subframe with truncation is indicated inthe associated downlink control information (DCI, which may be part ofthe PDCCH), that assigns the PDSCH. For example, the set of symbolss={0, 3, 5, 9} can be configured by higher layers such that two bits inthe DCI can signal one of the four values {0, 3, 5, 9} as being thenumber of symbols (m) in the truncated portion 404. In another example,a set {0, 3} can be configured by the eNB such that one bit in the DCIindicates whether the subframe is not truncated {0} or has a truncatedportion of 3 symbols (m=3). In yet another example, one bit can beconfigured in the DCI to indicate whether the subframe is or is not asubframe with truncation. Additionally, the UE can be configured byhigher layers (e.g., a RRC message) with a truncated symbol valuecorresponding to the number of truncated symbols that are part of thetruncated portion at the end of a subframe with truncation. When symbolsare truncated, the truncated portion is comprised of contiguous or a setof serial symbols.

A second example of an approach or method for signaling a UE 106 forsubframes with truncation 406 can be performed with a physical downlinkcontrol channel (PDCCH) or an enhanced physical downlink control channel(EPDCCH). Here the DCI contents of a PDCCH/EPDCCH transmitted in thecommon search space of the primary cell (Pcell, i.e., a cell operatingon the licensed carrier) indicate the number of symbols in the truncatedportion of the corresponding subframe with truncation of the Scell(operating on the unlicensed carrier). In this approach, thePDCCH/EPDCCH can be decoded by all UEs and/or a set of UEs operating onthe unlicensed carrier. A special radio network temporary identifier(RNTI) may be configured via higher layers for scrambling the cyclicredundancy check (CRC) of the associated DCI.

In a variant of the second example, the DCI contents of a PDCCH/EPDCCHtransmitted in the UE-specific search space/carrier-specific searchspace of another cell (e.g., a primary or secondary cell operating onthe licensed carrier) indicate the number of symbols that are truncated(i.e., a truncation value) in the truncated portion at the end of thesubframe (i.e., the terminal part of the subframe) in the correspondingsubframe with truncation of the Scell (operating on the unlicensedcarrier). In this approach, the PDCCH can be decoded by multiple UEsoperating on the unlicensed carrier for which the DCI is intended.

A third example of an approach or method for signaling a UE that certainsubframes are subframes with truncation and indicate which symbols ofthe subframe with truncation are truncated as part of the truncatedportion can be accomplished as follows. Instead of indicating thetruncation value, the higher layers configure the UE with one or moresubframe types, each subframe type may correspond to a subframe having apredetermined number of symbols in the subframe, e.g., subframe type 1corresponds to 14 OFDM symbols (i.e., 1 ms duration) and using a firsttype of reference signal structure, and subframe type 2 corresponds to11 OFDM symbols (11/14 ms duration) corresponding to a subframe withtruncation having a truncated portion of 3 ODFM symbols and using asecond type of reference signal structure, and so on.

In some scenarios, it is not possible for the eNB to truncate only thelast symbols (i.e., the terminal portion) of a subframe with truncation.For example, if the eNB begins a LBT clear channel assessment in thelast symbols of a subframe with truncation (e.g., due to limitation onthe maximum channel occupancy time), and if the carrier is not detectedas free until sometime in the next subframe (i.e., the subframeimmediately following a first subframe with truncation), then the eNBcan only start LTE transmission somewhere in the middle of the nextsubframe.

FIG. 5 illustrates another subframe structure 500 used for communicationbetween a base station and a UE. Here the end or terminal portion 504 ofsubframe n+2 and some of the beginning portion of subframe (n+3) aretruncated by the eNB into the truncated portion 502. In such a scenarioit is not possible for the eNB to indicate which symbols are truncatedsymbols in the beginning portion 506 of subframe (n+3) via controlsignaling 520 in subframe n+2 (because eNB will not know how long theLBT will take) or the control signaling in subframe (n+3) (because theLBT occurs before the subframe (n+3) signaling).

One option for the eNB 104 in this scenario is to truncate or nottransmit in the entire subframe (n+3) and then begin PDCCH/PDSCHtransmission in the following subframe after subframe (n+3) (i.e.,subframe (n+4)). The eNB may use some part of the truncated portion ofsubframe (n+3) to transmit a ‘carrier holding signal’ after the carrieris detected to be free in the truncated portion. However, this option isinefficient as all resources in subframe (n+3) are wasted (especially iftruncation is required only for a few beginning symbols of subframe(n+3)).

A more efficient option is to initiate control signaling transmission insubframe (n+3) after the eNB determines that the carrier is free. Withthis option, if the eNB uses PDCCH to assign PDSCH resources to a UE insubframe (n+3), and if some or all of the symbols of the first slot 510of the subframe (n+3) have to be truncated (e.g., due to an ongoingLBT), then the eNB can transmit the PDCCH 516 starting from the firstsymbol of the second slot 512. In this example, each subframe 508 in thesubframe structure 500 comprises 14 symbols wherein the first slot 510of a subframe 508 contains the first 7 symbols (s1-s7) and the secondslot 512 of the subframe 508 contains the second 7 symbols (s8-s14). Aslot boundary 514 is shown as a dashed horizontal line in each subframe508.

FIG. 5 illustrates subframe (n+3) where PDSCH 518 is transmitted in thesymbols in a second slot following PDCCH 516 because the first slot 510of the (n+3) subframe is part of the truncated portion 506 at thebeginning of subframe (n+3).

Alternatively, in some circumstances PDSCH may span both symbols in thesecond slot 512 of a first subframe with truncation (n+2) and somesymbols (e.g., the last two symbols or at least some symbols followingthe instant in time when the carrier is detected to be free towards theend of the first slot) in the first slot of a second subframe withtruncation (n+3) that immediately follows the first subframe withtruncation (n+2). For example, if the first 5 symbols in the first slot510 of the second subframe with truncation (n+3) are truncated, PDCCH516 is transmitted in the first symbol of the second slot 512 (i.e., thes8 symbol position in the second subframe with truncation (n+3)) andPDSCH 518 is assigned in the last two symbols of the first slot (6^(th)and 7^(th) symbol positions s6 and s7) of the second subframe withtruncation (n+3) and symbols in the second slot 512 of the secondsubframe with truncation (n+3). This alternative has the advantage ofutilizing the subframe resources a little more efficiently (assuming thefirst 5 symbols (m=5 and s=s1 through s5) of the subframe are enough fortruncation) and it is more suitable for PDSCH assignments fortransmission modes 8, 9, 10 that utilize demodulation reference signalspresent in the s6 and s7 symbol positions of the first slot of thesubframe.

Additional efficiency may be provided wherein, the DCI of the PDCCH 516in the subframe with truncation (n+3) can also indicate the number oftruncated symbols in that subframe at the beginning of the subframe withtruncation (n+3) so that PDSCH 518 is transmitted according to DCI(e.g., if DCI indicates 3 symbols (s1-s3), PDSCH 518 can be transmittedin the remaining 11 symbols (s4-s14)).

From a UE perspective, a UE typically monitors for PDCCH candidates inthe first few symbols in the first slot 510 of a subframe 508 wheremonitoring implies attempting to blindly decode a PDCCH candidateassuming a particular set of CCEs (control channel elements) within theUE's PDCCH search space.

However, for the alternatives described above, the UE has to monitor forPDCCH candidates in the first few symbols of both the first slot 510 andthe second slot 512 of each subframe 508. More specifically, the UEperforms b1 blind decodes (BDs) in one or more symbols at the beginningof the first slot of a subframe with truncation and b2 additional BDs inone or more symbols at the beginning of the second slot of the subframewith truncation (b1 and b2 are integers). The UE may perform theadditional b2 BDs only if the first b1 BDs are not successful. For thisoperation, the UEs PDCCH search space comprises two parts, the firstpart comprising PDCCH candidates assuming sets of CCEs within the firstm1 symbols (1≤m1≤3) of the first slot of the subframe with truncationand a second part comprising sets of CCEs within the first m2 symbols(1≤m2≤3) of the second slot of the subframe with truncation.

To indicate the span (i.e., the number of symbols) of PDCCH 516transmission in the second slot, the eNB can also transmit a PCFICH inthe first symbol of the second slot of the second subframe withbeginning truncation (n+3). Alternatively, the span of PDCCH in thesecond slot 516 can be fixed or configured (e.g., signaled byhigher-layers (e.g., a RRC message)) to 1 or 2 symbols (m=1 or 2) toreduce UE complexity. In addition to PDCCH and PCFICH, PHICH may also betransmitted at the beginning of the second slot 512 of the subframe withbeginning truncation (n+3). The RE (resource element) locations forPCFICH can follow the same mapping as that of existing LTEspecifications (except for the change from first slot to second slot).Alternatively, additional higher layer signaling may help the UEdetermine the PCFICH and PDCCH RE locations. The PHICH configuration issignaled in PBCH for the existing specifications, whereas the PHICH forsecond slot may be separately configured.

For the purpose of PDCCH decoding, and PDSCH RE mapping (if PDCCH withPDSCH assignment for the UE is detected), the UE may assume that CRS ispresent in the second slot 512 of the subframe with truncation (n+3)even if that subframe with truncation is configured as an MB SFNsubframe.

Referring now to FIG. 6, if the eNB 104 uses EPDCCH to assign PDSCHresources to a UE in subframe (n+3), and if some or all of the symbolsof the first slot 606 of the subframe (n+3) have to be truncated (e.g.,due to ongoing LBT), then the eNB can transmit the EPDCCH starting froman alternative OFDM symbol location 607 that begins after the truncationperiod 602.

In principle, the starting symbol for EPDCCH transmission in subframe(n+3) can be dynamically varied by the eNB based on length of thebeginning portion 604 truncation period, at the beginning of thesubframe with truncation (n+3). However, this would result in very highUE complexity (since UE has to blindly try all possible startinglocations).

A simpler alternative is for the eNB 104 to signal one additional EPDCCHstarting location to the UE 106 via RRC signaling if necessary. Incurrent LTE systems, the EPDCCH starting symbol location is signaled tothe UE via RRC (e.g., it can be 2^(nd), 3^(rd) 4^(th) symbol in thefirst slot for a 20 MHz carrier). In various embodiments, the eNB willadditionally configure the UE with an additional EPDCCH startinglocation (e.g., 5^(th), 6th or 7^(th) symbol of the first slot 606, orthe first symbol of second slot 608) to accommodate alternative EPDCCHtransmission if the first few symbols of the first slot 606 of thesubframe with beginning truncation (n+3) are truncated.

An even simpler alternative (albeit less efficient than the above twoexamples) comprises transmitting EPDCCH starting from the first symbolof the second slot 608 if a higher layer configured starting symbol (inthe first slot 606) has to be truncated.

In various embodiments, PDSCH can be transmitted from the same startinglocation as that of EPDCCH or DCI of the EPDCCH, and can indicate thenumber of truncated symbols at the beginning of the subframe withtruncation (n+3) so that PDSCH is transmitted according to DCI (e.g., ifDCI indicates 3 truncated symbols, PDSCH can be transmitted in theremaining 11 symbols).

From a UE perspective, the UE performs b1 BDs assuming a first EPDCCHstarting position configured by higher layers, and performs b2additional BDs assuming an alternative EPDCCH starting position in asubframe. (Here b1 and b2 are integers.) The UE may perform theadditional b2 BDs only if the first b1 BDs are not successful. Thealternative EPDDCH starting position in a subframe may be fixed (e.g.,the s6 symbol position of first slot of each subframe), or it may beconfigured via RRC.

In another embodiment, the UE performs b1 BDs assuming EPDCCH iscontained within the 1^(st) slot, performs b2 additional BDs assumingEPDCCH is contained within the 1^(st) and 2^(nd) slot, and b3 BDs forEPDCCH contained within only 2^(nd) slot. (Here b1, b2 and b3 are eachintegers.) This is useful for cases where the UE does not know if thesubframe with truncation has a truncated portion at the beginning or theend of the subframe, i.e., the eNB has the flexibility to schedule theUE in 1^(st) slot only, 2^(nd) slot only or 1^(st) and 2^(nd) slottogether.

If the beginning portion of a subframe has to be truncated (e.g., asshown in subframe (n+3) in FIG. 5), another alternative is for the eNB104 to indicate the truncated beginning portion 506 of a given subframewith truncation (n+3) in the next subframe (n+4). For example, when thefirst few beginning symbols of subframe (n+3) shown in FIG. 5 aretruncated, then instead of making the UE determine the starting symbolof control channel transmission 516 in subframe (n+3) via the blinddecoding approaches described above, the eNB can signal the truncationinformation for subframe (n+3) using DCI of a PDCCH/EPDCCH transmittedin subframe (n+4) (not specifically shown). The DCI should haveadditional information indicating that the truncation informationapplies to the previous subframe (i.e., subframe n+3) and not thecurrent subframe (n+4). This can be provided by a subframe indicatorbit(s) in the DCI of subframe (n+4), or by using a configuredcross-subframe RNTI. The DCI can also provide resource allocationinformation for the previous subframe (i.e., subframe n+3).

In current LTE systems, for receiving PDSCH transmissions for sometransmission modes (e.g., TM 8, 9, or 10), the UE uses UE-specificdemodulation reference signals (UERS) present in REs of the last twosymbols of a subframe for LTE frame structure type 1 (FDD). However, forLTE operation on unlicensed carriers, in subframes where the last two ormore symbols are truncated, these UERS will be unavailable to the UE,and utilizing only the remaining UERS in other non-truncated symbolswill result in degraded channel estimation. Embodiments solve thisproblem by providing a more efficient alternative. That is, for LTEframe structure type 1 (FDD), for transmitting UE-specific referencesignals for antenna ports 7, 8, 9 and 10:

(1)—if the eNB has to provide a subframe with truncation wherein onlythe last three or fewer symbols in an end portion 608 in a subframe withtruncation (n+2) are truncated using normal cyclic prefix (CP), then forthose subframes, the eNB uses the UE-specific reference signals patterndescribed for LTE frame structure type 2 (TDD) special subframeconfigurations 3, 4, 8, or 9 in the 3GPP TS 36.211 vc.2.0 specification.

(2)—if the eNB has to truncate more than last three symbols in thesecond slot 610 of a subframe with truncation (n+2) using normal CP,then for that subframe with truncation (n+2), the eNB uses theUE-specific reference signals pattern described for LTE frame structuretype 2 (TDD) special subframe configurations 1, 2, 6, or 7 in the 3GPPTS 36.211 vc.2.0 specification.

This alternative is efficient as the UERS pattern for (TDD) specialsubframe configurations 3, 4, 8, or 9 in the 3GPP TS 36.211 vc.2.0specification does not include REs in the last three symbols of thesubframe. Similarly, the UERS pattern for (TDD) special subframeconfigurations 1, 2, 6, or 7 does not include REs in the second slot ofthe subframe.

The CSI-RS transmissions can also be adapted to fit into the subframeswith truncation. The OFDM symbols in which CSI-RS are transmitted areconfigured via higher layers. If an OFDM symbol in which CSI-RS is to betransmitted corresponds to a truncated OFDM symbol, then the UE mayassume that the CSI-RS is not transmitted in the OFDM symbol. In anotherapproach, the CSI-RS configuration for truncated subframes may beconfigured via separate higher layer signaling.

In existing LTE systems Scells are typically operated on a carrier (alsosometimes referred as a ‘component carrier’ or a ‘channel’) using thefollowing states:

(1) State where the Carrier is not Added as Scell (“not Configured”State):

Here the UE may be configured to perform inter-frequency measurements onthe carrier frequency layer using measurement gaps. No additional powerconsumption is necessary due to Scell receiver. The Scell receiver neednot be turned on and measurements can be performed using the Pcellreceiver.

(2) State where the Carrier is Added as Scell but not Activated(“Deactivated State”):

Here the UE receives relevant system information for the Scell from thePcell. The UE may be configured to perform measurements on the Scellwithout measurement gaps. There will be minimal additional powerconsumption due to Scell receiver. The Scell receiver needs to beoccasionally turned on (e.g., for 5 ms every 160 ms) for RRMmeasurements using either CRS or Discovery signals

(3) State where the Carrier is Added as Scell and Activated (“ActivatedState”):

Here the UE is expected to receive PDSCH on the Scell in all activatedsubframes. There is high additional power consumption due to the Scellreceiver. Scell receiver should perform PDCCH monitoring in everysubframe (for self scheduling case). The Scell receiver needs to bufferevery subframe for potential PDSCH processing (for both self andcross-carrier scheduling cases)

For Scell operation on unlicensed carriers, it is desirable to retainthe above states so that UE power consumption for unlicensed carrieroperation is similar to that of licensed carrier operation.

However, it should be noted that some of the embodiments, when comparedto current LTE operation, are further required to enable efficientoperation taking into account the need for the eNB to perform LBT andobey discontinuous transmission requirements (DCT requirements).

FIG. 7 illustrates an example of Scell operation on an unlicensedcarrier frequency. In FIG. 7, the eNB operates (i.e., performs thenecessary steps to communicate with a UE) a Pcell 700 on a licensedcarrier. The eNB also operates a Scell 702 (denoted as uScell in FIG. 7)on an unlicensed carrier. The UE receives transmissions from the eNB onboth the Pcell and uScell 704 (only uScell reception is shown).Subframes indicated as “D” represent deactivated state subframes for theUE. Subframes indicated as “A” represent activated state subframes forthe UE.

When a data burst to be transmitted on the uScell arrives in the eNBbuffer 701, the eNB starts LBT 706 on the unlicensed carrier. Afterdetermining that the carrier is free 708, the eNB starts transmitting712 a reserving transmission on the unlicensed carrier to effectivelyreserve the unlicensed carrier frequency. The reserving transmission ismade to ensure that other devices sensing the carrier detect that thecarrier is occupied by the eNB. It also prepares an activation command710 for transmission on the PCell to the UE. When the eNB is ready totransmit the activation command (e.g., after a one or two subframe delayfor internal processing, two subframe delay shown in the figure), ittransmits the activation command 710 on the Pcell. The UE, afterreceiving the activation command 710, transitions to an unlicensedcarrier frequency receive activated state A from its deactivatedunlicensed carrier receive state D. The UE may need a few subframes(e.g., 1 or 2 subframes, 1 subframe shown in the figure) to make thistransition 714. Once the UE is in activated state, the eNB schedulesdata (using PDSCH) to the UE. After transmitting for a few subframes(e.g., 5 subframes shown in the figure), the eNB stops transmission andreinitiates LBT 715 on the unlicensed carrier to satisfy DCTrequirements. After the carrier is determined to be free 716, the eNBreinitiates PDSCH transmission to the UE. This process is continued tillthe data burst transmission is complete.

Preparation of activation command at the eNB includes determining DCIpayload bits, CCE locations and CCE aggregation level of the PDCCHassociated with the activation command. Some of the steps for preparingthe activation command can be performed in advance by the eNB (e.g.,determining payload bits). However, other steps (e.g., determining CCElocations for PDCCH transmission) can only be performed afterdetermining the subframe where the activation command is transmitted730, which in turn, depends on the subframe in which the eNB determinesthe unlicensed carrier is free 732. For example, if the eNB determinesthat the unlicensed carrier is free in subframe n, it prepares foractivation command transmission on the Pcell in subframe (n+3). Asexplained earlier, between subframe (n) and (n+3), the eNB continuestransmitting on the unlicensed carrier 712 to ensure that other devicessensing the carrier realize that the carrier is occupied by the eNB. Thetransmissions may include reference signal transmissions such as CRS orCSI-RS.

To shorten the UE transition 714 from deactivated D to activated Astate, the activation command can be embedded in physical layersignaling (e.g., within the DCI payload of a PDCCH), instead of thehigher layer MAC CE based approach (which involves both PDCCH and PDSCH)used in current LTE systems.

The eNB has to transmit at least reference signals (e.g., CRS) on theunlicensed carrier in the subframe 734 where the activation command istransmitted and a few immediately following subframes (e.g., onesubframe 735 following the activation command). These reference signalscan be used by the UE (that is being activated) for purposes such asregaining fine time/frequency sync and automatic gain control (AGC) warmup.

From the UE perspective, it continues to be in activated state A untilit receives a deactivation command from the eNB or the expiration of adeactivation timer configured by the eNB via higher layers (whicheveroccurs earlier) 720, after which the UE transitions 718 to the low powerdeactivated state.

As shown in the FIG. 7, for unlicensed carrier operation, the eNB maynot have any transmissions in some subframes 736 (e.g., due to LBTprocedure related delay) when the UE is in activated state. This isdifferent from current LTE systems where activated UEs expect the eNB totransmit at least CRS in every subframe. The UEs need to receivefrequent reference signal transmissions from the eNB (e.g., at least onetransmission every X=10 or 20 ms) for AGC maintenance and finetime/frequency tracking.

For unlicensed carrier operation, the eNB may use any one of thefollowing approaches to ensure that activated UEs have enough referencesignals for AGC maintenance and fine time/frequency tracking. Forexample, in one embodiment for unlicensed carrier operation, the eNB maytransmit discovery signals more frequently than what is consideredstandard. More particularly, the eNB may transmit discovery signals onceevery 10 ms to 20 ms. However, this embodiment's approach may mostlikely lead to increased interference on the unlicensed carrierfrequency and increased eNB power consumption since the transmitteddiscovery signals are typically always transmitted by the eNB tofacilitate cell detection.

In another embodiment, when the eNB determines that it has not made anytransmissions on an unlicensed carrier for some time (e.g., for about 10to 20 ms), the eNB will not schedule PDSCH to an activated UE on thatcarrier, until the eNB transmits a few subframes (e.g., 1 or 2subframes) containing at least reference signals (e.g., CRS), and thenresume PDSCH transmissions to the UE thereafter. Here, the eNB mayoptionally send an indicator on the PCell (e.g., special PDCCH/EPDCCHmessage) that tells the UE whether reference signal transmissions (e.g.,CRS) are present on the Scell for a particular subframe or a set ofsubframes.

In other embodiments for unlicensed carrier operation, the eNB and theactivated UE can maintain a DL time synchronization timer. The DL timesynchronization timer can be set to a value Xms (e.g., X=10 or 20 ms,the value of X depends on time for which UE can autonomously maintaintime-frequency sync without receiving any eNB transmissions). Thesubframe in which the DL time synchronization timer is set to Xms can beany subframe in which the eNB transmits PDSCH to the UE (from UEperspective, it will be any subframe where PDSCH is received from theeNB). Alternately, the subframe can be any subframe where the UEtransmits a positive acknowledgement (HARQ-ACK) in response to a PDSCHreceived from the eNB (in this case, from eNB perspective, it will beany subframe in which a positive acknowledgement is received from theUE). The DL time synchronization timer is then counted down insubsequent subframes. When the DL time synchronization timer expires(i.e., when it becomes 0), the eNB determines that the UE may not havefine time frequency synchronization. Although, the UE may not have finetime frequency synchronization, the UE can continue to monitor Scellsubframes for control signaling, as it may be able to decode somePDCCH/EPDCCH and/or PDSCH messages (albeit with a higher bit error rate)When the eNB determines the UE may not have fine time frequencysynchronization, the eNB can schedule PDSCH transmissions with aconservative PDSCH assignment (e.g., using MCS corresponding to QPSK andone spatial layer instead of 64QAM and multiple spatial layers), for afew subframes (e.g., 1 or 2 subframes) until the UE regains finetime/frequency sync to decode more aggressive PDSCH assignments. Thisalternative avoids the overhead of re-activation command transmission onthe PCell (described below). In the alternative, the eNB can send anactivation command (in this example, a re-activation command) to the UEon the PCell. Upon reception of the activation command on the Pcell insubframe n, the UE determines that reference signal transmissions arepresent in at least subframes (n) to n+p (e.g., p=1 or 2) and regainsfine time-frequency sync using the signals in those subframes.

In yet another embodiment for unlicensed carrier frequency operation,the eNB may also reuse a deactivation timer as a ‘DL timesynchronization timer’ described above (i.e., the eNB configures the UEwith a deactivation timer of Xms, and resends the activation commandwhenever the timer expires (if there is still data in the buffer fortransmission to that UE)). In this embodiment situation, the UEdeactivates the Scell when the timer (deactivation timer/DL timesynchronization timer) expires.

Although the preferred embodiments have been described herein in detail,it should be understood that various changes, substitutions andalterations can be made therein without departing from the teachings,understanding and scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. A method for a base station to make atransmission between the base station and a user equipment (UE) on anunlicensed carrier frequency, the method comprising: operating, by thebase station, a first serving cell on a licensed carrier frequency and asecond serving cell on an unlicensed carrier frequency; performing, bythe base station, a listen before talk (LBT) on the unlicensed carrierfrequency for a first time duration; determining, by the base station,that the unlicensed carrier frequency is free; transmitting, by the basestation, an activation command on the first serving cell for receipt bythe UE, the activation command being configured to activate the UE toreceive transmissions from the base station on the second serving cell;and transmitting by the base station, after determining that theunlicensed carrier frequency is free, a reserve transmission on theunlicensed carrier frequency at least until the base station beginstransmission of the activation command on the first serving cell.
 2. Themethod of claim 1, further comprising, the base station preparing theactivation command while transmitting the reserve transmission on theunlicensed carrier frequency.
 3. The method of claim 1, furthercomprising scheduling, by the base station, data to be transmitted tothe UE on the unlicensed carrier frequency for a determined number ofsubframes.
 4. The method of claim 3, further comprising: discontinuingtransmission of scheduled data by the base station to the UE after thedetermined number of subframes; and performing, by the base station, theLBT on the unlicensed carrier frequency for a second time duration. 5.An apparatus comprising: a controller that operates a first serving cellon a licensed carrier frequency and a second serving cell on anunlicensed carrier frequency, performs a listen before talk (LBT) on theunlicensed carrier frequency for a first time duration, and determinesthat the unlicensed carrier frequency is free; and a transceiver coupledto the controller, where the transceiver transmits an activation commandon the first serving cell for receipt by a user equipment (UE), theactivation command being configured to activate the UE to receivetransmissions from the base station on the second serving cell, andtransmits, after determining that the unlicensed carrier frequency isfree, a reserve transmission on the unlicensed carrier frequency atleast until the base station begins transmission of the activationcommand on the first serving cell.
 6. The apparatus of claim 5, whereinthe controller prepares the activation command while transmitting thereserve transmission on the unlicensed carrier frequency.
 7. Theapparatus of claim 6, wherein the controller schedules data to betransmitted to the UE on the unlicensed carrier frequency for adetermined number of subframes.
 8. The apparatus of claim 7, wherein thecontroller discontinues transmission of scheduled data by the basestation to the UE after the determined number of subframes, and performsthe LBT on the unlicensed carrier frequency for a second time duration.